1. Technical Field
The present invention relates to a semiconductor device.
2. Related Art
Semiconductor devices so far developed include those disclosed in JP-A No. H11-297837, JP-A No. 2005-39220, and JP-A No. 2005-57186. Those semiconductor devices include a fuse. In such semiconductor devices, disconnecting the fuse enables some operations such as adjusting a resistance employed in the semiconductor device and invalidating a defective element to substitute with a normal element. The technique of substituting a defective element with a normal element is utilized, for example, for securing redundancy of a semiconductor storage unit.
Methods of disconnecting the fuse include laser irradiation and loading a current. The former is disclosed in JP-A No. H11-297837, and the latter in JP-A No. 2005-39220 and JP-A No. 2005-57186.
FIG. 7 is a circuit diagram of the semiconductor device according to JP-A No. H11-297837. FIG. 7 shows a signal output unit 100 that outputs a binary signal depending on the disconnection status of the fuse, and an internal circuit 110 which is the object of substitution. The signal output unit 100 includes three fuses 101, 102, and 103, so that a NAND gate 107 outputs a high level (hereinafter, H-level) signal when at least one of the fuses 101, 102, and 103 is disconnected. Specifically, each of the fuses 101, 102, and 103 has an end connected to a power supply terminal 108, and the other end connected to an input terminal of the NAND gate 107. Also, the other ends of the fuses 101, 102, and 103 are grounded via a pull-down resistance 104, 105, and 106, respectively.
In the internal circuit 110, the output of the NAND 107 is provided to transfer gates 112 and 113 via an inverter 111. Thus, two address buses 114 and 115 are mutually switched according to the output of the NAND 107.
In the circuit shown in FIG. 7, the desired result from disconnecting the fuse (for example, substitution of the defective element with the normal element) can be obtained, if at least one of the three fuses 101, 102, and 103 is disconnected. Accordingly, there is less likelihood that incomplete disconnection of the fuses 101, 102, and 103 affects the output of the signal output unit 100. In other words, the complete disconnection of the fuse is substantially further assured.
However, the fuse once disconnected may be reconnected with the lapse of time. Accordingly, reducing the likelihood that such change over time affects the output of the signal output unit is also essential in providing a reliable semiconductor device.